feat(store): migrate to the anchored model, schema 8

Strips occurrenceDate from an anchored cost's charges, because that path has
no slots and a leftover slot id is a lie a future reader would trust. Expands
a month-only startDate on an anchored monthly cost to the 1st, because day
precision now needs a real day. calendar costs are not touched at all.

Deliberately NOT behaviour-preserving — an anchored cost's projected dates
change, which is the point. Every recorded figure is preserved: no Money is
touched, no purchase added or removed.

src/domain/seed.ts

@@ -1,7 +1,7 @@
 import { DEFAULT_DIGITS, SEED_CURRENCIES } from "./types.ts";
 import type { CurrencyDef, Dataset, FxRate, MonthId, Post } from "./types.ts";
 
-export const SCHEMA_VERSION = 7;
+export const SCHEMA_VERSION = 8;
 
 const SEED_POST_NAMES = ["Video Games", "Food", "Events and Social"] as const;
 

src/store/migrations.test.ts

@@ -485,3 +485,72 @@ describe("6 -> 7: recurring costs join the dataset", () => {
     }
   });
 });
+
+describe("7 -> 8: the anchored model", () => {
+  function v7(recurring: any[], purchases: any[]): any {
+    const base = createSeedDataset("2026-09");
+    return {
+      ...base,
+      settings: { ...base.settings, schemaVersion: 7 },
+      recurring,
+      purchases,
+    };
+  }
+  const anchored = {
+    id: "phone", name: "Phone", order: 0, archived: false,
+    amount: { amount: 300, currency: "DKK" },
+    startDate: "2026-09", recurrence: { kind: "everyNMonths", n: 1 },
+    anchoring: "lastCharge", splitMode: "percent",
+    splits: [{ postId: "p", value: 100, absorbsRemainder: true }],
+  };
+  const grid = { ...anchored, id: "rent", name: "Rent", anchoring: "calendar" };
+
+  function purchase(recurringId: string, occurrenceDate: string) {
+    return {
+      id: `pay-${recurringId}`, date: "2026-09-20", description: "x",
+      total: { amount: 300, currency: "DKK" }, splitMode: "percent",
+      splits: [{ postId: "p", value: 100, absorbsRemainder: true }],
+      schedule: null, source: { recurringId, occurrenceDate },
+    };
+  }
+
+  test("strips occurrenceDate from an ANCHORED cost's charges", () => {
+    const out = migrate(v7([anchored], [purchase("phone", "2026-09")]));
+    expect(out.purchases[0]!.source).toEqual({ recurringId: "phone" });
+    expect(out.settings.schemaVersion).toBe(SCHEMA_VERSION);
+  });
+
+  test("LEAVES occurrenceDate on a calendar cost's charges", () => {
+    const out = migrate(v7([grid], [purchase("rent", "2026-09")]));
+    expect(out.purchases[0]!.source).toEqual({ recurringId: "rent", occurrenceDate: "2026-09" });
+  });
+
+  test("expands a month-only startDate on an anchored MONTHLY cost", () => {
+    // Day precision now needs a real day; the 1st is the honest reading of
+    // "this cost started in September".
+    expect(migrate(v7([anchored], [])).recurring[0]!.startDate).toBe("2026-09-01");
+  });
+
+  test("leaves a calendar monthly cost's month-only startDate alone", () => {
+    expect(migrate(v7([grid], [])).recurring[0]!.startDate).toBe("2026-09");
+  });
+
+  test("leaves an already day-granular startDate alone", () => {
+    const dated = { ...anchored, startDate: "2026-09-20" };
+    expect(migrate(v7([dated], [])).recurring[0]!.startDate).toBe("2026-09-20");
+  });
+
+  test("an ordinary purchase with no source is untouched", () => {
+    const plain = { ...purchase("phone", "2026-09"), source: undefined };
+    delete (plain as any).source;
+    const out = migrate(v7([anchored], [plain]));
+    expect(out.purchases[0]!.source).toBeUndefined();
+  });
+
+  test("NO recorded figure changes", () => {
+    const out = migrate(v7([anchored], [purchase("phone", "2026-09")]));
+    expect(out.purchases[0]!.total).toEqual({ amount: 300, currency: "DKK" });
+    expect(out.recurring[0]!.amount).toEqual({ amount: 300, currency: "DKK" });
+    expect(out.purchases.length).toBe(1);
+  });
+});

src/store/migrations.ts

@@ -193,6 +193,44 @@ const MIGRATIONS: Array<(data: any) => any> = [
     settings: { ...data.settings, schemaVersion: 7 },
     recurring: Array.isArray(data.recurring) ? data.recurring : [],
   }),
+
+  // 7 -> 8: recurring costs split into two projection models.
+  //
+  // A `lastCharge` cost no longer has a grid of slots — its next due date is
+  // one cycle after the latest actual charge — so `occurrenceDate` on its
+  // charges is meaningless, and leaving it would invite a future reader to
+  // trust it. A `calendar` cost keeps its slots and is not touched.
+  //
+  // Day precision also now requires a real day, so an anchored MONTHLY cost
+  // whose startDate was month-only is expanded to the 1st.
+  //
+  // NOT behaviour-preserving, and it must not pretend to be: an anchored
+  // cost's projected dates deliberately change, which is the point of the
+  // change. What IS preserved is every recorded figure — no Money is touched,
+  // no purchase is added or removed.
+  //
+  // The two literals below are inline strings, not imported constants: this
+  // step must not read a live default.
+  (data: any) => {
+    const recurring = Array.isArray(data.recurring) ? data.recurring : [];
+    const anchoredIds = new Set(
+      recurring.filter((c: any) => c?.anchoring === "lastCharge").map((c: any) => c?.id),
+    );
+    return {
+      ...data,
+      settings: { ...data.settings, schemaVersion: 8 },
+      recurring: recurring.map((cost: any) => {
+        if (cost?.anchoring !== "lastCharge") return cost;
+        if (cost?.recurrence?.kind !== "everyNMonths") return cost;
+        if (typeof cost.startDate !== "string" || cost.startDate.length !== 7) return cost;
+        return { ...cost, startDate: `${cost.startDate}-01` };
+      }),
+      purchases: (Array.isArray(data.purchases) ? data.purchases : []).map((p: any) => {
+        if (!p?.source || !anchoredIds.has(p.source.recurringId)) return p;
+        return { ...p, source: { recurringId: p.source.recurringId } };
+      }),
+    };
+  },
 ];
 
 export function migrate(raw: unknown): Dataset {